Update version of go in base image

[bobcatfish]

Expected Behavior

Our unit tests as run by Prow (which is configured to use gcr.io/tekton-releases/tests/test-runner@sha256:a4a64b2b70f85a618bbbcc6c0b713b313b2e410504dee24c9f90ec6fe3ebf63f ) should catch race conditions such as tektoncd/pipeline#1124.

Actual Behavior

The unit tests are run with -race (you can find the code for this somewhere in library.sh in the vendored scripts) but they don't catch the issue. I'm guessing this is b/c of the version of go inside the image, but not sure:

docker run --entrypoint go gcr.io/tekton-releases/tests/test-runner@sha256:a4a64b2b70f85a618bbbcc6c0b713b313b2e410504dee24c9f90ec6fe3ebf63f version      
go version go1.11.4 linux/amd64

Steps to Reproduce the Problem

1. Checkout tekton pipelines before the fix in Fix and work around timeout handler data races pipeline#1308
2. Run go test ./... -race - see the data race detected
3. Run the tests in the image we use with prow instead, e.g.:

# from the dir you've checked out tekton pipelines into
docker run -it -v `pwd`:/go/src/github.com/tektoncd/pipeline/ --entrypoint /bin/bash gcr.io/tekton-releases/tests/test-runner@sha256:a4a64b2b70f85a618bbbcc6c0b713b313b2e410504dee24c9f90ec6fe3ebf63f
$ cd /go/src/github.com/tektoncd/pipeline/
$ go test ./... -race

See that there is no data race!